From the Members Area > Selling > Seller Tools > Default Listing Settings page, you can set up the default settings to use when you create new listings. However, these settings will only affect new listings you create, and will not affect your existing listings.
If instead (or in addition) you want to bulk edit / update your existing listings, you can easily do this from the Members Area > Selling > Listings page. At the top left of the page, click on the drop-down menu connect to the first checkbox, and then select the "Select All XX Listings" option:
Then scroll to the bottom of the page and click on the "Bulk Edit" button. Once the Bulk Edit window appears, you will see most of the same fields you would when creating a new listing, or editing an individual listing; such as Shipping Costs, Enabling Offers, etc.
Click "Edit" next to and field you wish to change. Review your changes, and then click the "Submit" button.
Your listings will then be updated over the next few minutes with the changes you've indicated.